This application contains subject matter described and claimed in U.S. patent application Ser. No. 304,173 filed on Nov. 6, 1972 and U.S. patent application Ser. No. 312,011 filed on Dec. 14, 1972, said applications being assigned to the assignee of the present invention and which issued as U.S. Pat. No. 3,840,253 on Apr. 8, 1974, and U.S. Pat. No. 3,796,028 on Mar. 12, 1974, respectively.